Millimeter wave polarimetric radar remote sensing of ice clouds.

摘要

Ice clouds play important roles in many practical and theoretical researches. This thesis investigates the electromagnetic scattering properties of ice crystals at 94 and 220 GHz, and polarimetric radar techniques for ice crystal type discrimination and ice mass content estimation.; The scattering amplitude matrix is computed for pristine ice crystals of different sizes and from different incidence directions using the Finite Difference Time Domain method. Hexagonal plates, stellar crystals, and hexagonal columns with empirical aspect ratios are considered. The results show that the co-polarized scattering amplitudes are not sensitive to the azimuthal incidence angle but dependent on the polar incidence angle {dollar}theta{dollar} as functions of {dollar}costheta{dollar} or {dollar}sintheta{dollar} raised to a power which depends on particle size. Cross-polarized scattering amplitudes are negligible when the wave polarization is aligned with respect to the particle symmetry axis.; Numerical computations are performed to examine the dependence of polarimetric radar parameters on the parameters in the gamma size and Gaussian canting angle distributions, and on radar elevation angle. The computed Mueller matrix elements related to the cross-correlation of the co-polarized and cross-polarized scattering amplitudes are less than 5% of the total irradiance. The linear depolarization ratio, circular depolarization ratio, and dual-frequency ratio are found depolarization ratio, circular depolarization ratio, and dual-frequency ratio are found useful for differentiating between planar ice crystals and columns.; Five relationships between ice mass content and polarimetric radar parameters are derived based on numerical simulations representing various assumed ice mass contents and gamma size distributions. The specific differential phase at incidence angles away from the zenith, and effective reflectivity factor together with dual-frequency ratio can provide reasonable estimates for ice mass content.; Simulations based on in situ two-dimensional cloud particle image probe measurements are compared with measurements by a 95 GHz polarimetric radar for an ice crystal cloud. The results for Gaussian canting angle distribution with standard deviation equal to 40{dollar}spcirc{dollar} match well with the radar measurements. Sensitivity tests show that radar parameters are affected in varying degrees by ice crystal density, aspect ratio, canting angles, and effects of small particles.